1. Field of the Invention
The present invention relates to a mobile communication system that carries out digital communication with efficient use of vacant channels in a one-way communication which is performed by separately assigning different channels for the transmission and the receiving. The present invention relates to a method for channel assignment of the mobile communication system. Further, the present related to a control unit for use in the mobile communication system.
2. Description of the Related Art
A mobile communication system that is designed to carry out public service operations, such as fire or police call management operations, by using a multi-channel access (MCA) technique is known. FIG. 20 shows a conventional mobile communication system of this type.
In the conventional mobile communication system of FIG. 20, mobile station units 10, base station units 20, and a central control unit 30 are provided. Each of the mobile station units 10 is formed by mounting a radio communication device on a fire engine or the like. The base station units 20 and the mobile station units 10 communicate with each other by using radio channels. The central control unit 30 controls the radio channels of the base station units 20 and is linked to a core network L.
Generally, in the mobile communication system, a multiple access communication is performed by using the frequency division multiple access (FDMA) method or the time division multiple access (TDMA) method. Alternatively, the code division multiple access (CDMA) method may be used to perform the multiple access communication.
In the above conventional mobile communication system, a two-way communication between the mobile station units 10 is normally performed. In the central control unit 30, upstream and downstream communication channels are assigned to each of the respective mobile station units 10 that perform the two-way communication, in order to achieve the two-way communication between the mobile station units 10. Another type of the one-way communication between the mobile station units 10, which is carried out in the conventional mobile communication system of FIG. 20, is a one-to-multiple one-way communication in which a single transmitter sends information to multiple receivers.
A description will be given of the one-to-multiple one-way communication (hereinafter, called the group communication) between the mobile station units 10 with reference to FIG. 20.
Suppose that, in the mobile communication system of FIG. 20, the FDMA method is used to carry out the group communication. In the FDMA method, the available channel bandwidth is divided into a number of frequency non-overlapping subchannels and a subchannel is assigned to each mobile station unit 10 upon request by the mobile station units 10.
As shown in FIG. 20, the mobile station units 10-1 and 10-2 exist in a radio zone xe2x80x9cZoneAxe2x80x9d of the base station unit 20-1, and the mobile station units 10-3 and 10-4 exist in a radio zone xe2x80x9cZoneBxe2x80x9d of the base station unit 20-2. In the mobile communication system of FIG. 20, the mobile station units 10-1 through 10-4 pertain to the same group (called group 1) for the group communication.
When a request for the group communication related to the group 1 is transmitted by the mobile station unit 10-1, the central control unit 30 receives the group communication request via the base station unit 20-1. In response to the request, the central control unit 30 retrieves the radio zone in which the mobile station units pertaining to the group 1 exist, by accessing, through the core network, a home location register (HLR) 50 that is connected to the core network. As described above, the mobile station units 10-2, 10-3 and 10-4 pertain to the group 1, the mobile station unit 10-2 exists in the radio zone xe2x80x9cZoneAxe2x80x9d of the base station unit 20-1, and the mobile station units 10-3 and 10-4 exist in the radio zone xe2x80x9cZoneBxe2x80x9d of the base station unit 20-2.
FIG. 21 shows a channel assignment of the conventional mobile communication system of FIG. 20 in which radio channels are assigned to the mobile stations.
As shown in FIG. 21, in the above case of the group communication request, the central control unit 30 assigns an upstream subchannel f1 (channel1) and a downstream subchannel f2 (channel2) for the radio zone ZoneA of the base station unit 20-1, and assigns an upstream subchannel f3 (channel3) and a downstream subchannel f4 (channel4) for the radio zone ZoneB of the base station unit 20-2. This channel assignment is the same as that in the case of the normal two-way communication.
During the group communication between the mobile station units, the mobile station unit 10-1 transmits an upstream group-communication signal at the frequency f1 to the central control unit 30, whereas the central control unit 30 transmits a downstream group-communication signal at the frequency f2 to the mobile station unit 10-2 and transmits a downstream group-communication signal at the frequency f4 to the mobile station units 10-3 and 10-4. Hence, the one-to-multiple one-way communication between the mobile station units 10 (or the group communication) is carried out by the conventional mobile communication system.
In the TDMA method, the multiple subchannels for multiple access are created by dividing the frame duration into a number N of non-overlapping subintervals, each of individual durations divided by the number N. Then each mobile station unit that requests the group communication is assigned to a particular time slot within each frame. In the CDMA method, each mobile station unit is assigned a unique code sequence that allows the mobile station unit to spread the information signal across the assigned frequency band. Thus, the multiple subchannels for multiple access are created.
However, the conventional mobile communication system assigns the subchannels to the mobile station units in the group communication (the one-way communication) in a manner that is the same as in the normal two-way communication. According to the channel assignment of the conventional mobile communication system, the subchannels that are not used for the group communication are also assigned to the mobile station units. As a result, the efficiency of use of the available frequency channels is lowered.
As described above, the conventional mobile communication system assigns both the upstream subchannel and the downstream subchannel for each base station unit 20 during the group communication. In the channel assignment of FIG. 21, the upstream subchannel f3 is assigned for the base station unit 20-2, but only the downstream subchannel f4 is used to transmit the group-communication signal to the mobile station units 10-3 and 10-4 during the group communication. Therefore, during the group communication of the conventional mobile communication system, the vacant channels (for example, f3) are always provided in the base station units 20 that are different than the base station unit 20 including the mobile station unit 10 transmitting the group communication request and include the mobile station units 10 related to the group specified by the request.
Further, in the conventional mobile communication system of FIG. 20, the mobile station unit 10-2 pertains to the base station unit 20-1 that includes the mobile station unit 10-1 transmitting the group communication request, and is included in the radio zone of the base station unit 20-1. In this case, the downstream subchannel f2 is assigned to the mobile station unit 10-2 in the base station unit 20-1, and the downstream subchannel f2 is used to transmit the group-communication signal to the mobile station unit 10-2. However, there is a case in which the mobile station unit 10 related to the group specified by the group communication request is not included in the radio zone of the base station unit 20-1 including the mobile station unit 10-1 transmitting the group communication request. In the latter case, the downstream subchannel is assigned to that mobile station unit 10, but the downstream subchannel is not used to transmit the group-communication signal to that mobile station unit 10. As a result, the efficiency of use of the available frequency channels is lowered.
Accordingly, it is an object of the present invention to provide an improved mobile communication system in which the above-described problems are eliminated.
Another object of the present invention is to provide a mobile communication system in which a one-way communication is carried out with efficient use of the vacant channels as in the group communication performed by the conventional mobile communication system.
Another object of the present invention is to provide a method for channel assignment of a mobile communication system in which a one-way communication is carried out with efficient use of the vacant channels as in the group communication performed by the conventional mobile communication system.
Another object of the present invention is to provide a control unit for use in a mobile communication system, which is configured to carry out a one-way communication with efficient use of the vacant channels as in the group communication performed by the conventional mobile communication system.
According to one preferred embodiment of the present invention, a mobile communication system in which pairs of upstream and downstream channels are assigned to mobile station units located in a radio zone of a base station unit to carry out a two-way communication between the mobile station units, and pairs of upstream and downstream channels are assigned to the mobile station units to carry out a one-to-multiple group communication between the mobile station units, includes: a detection unit which detects a vacant communication channel that exists with respect to the group communication carried out in the system; and a channel assignment unit which assigns the vacant communication channel, detected by the detection unit, for a one-way communication different from the group communication.
According to one preferred embodiment of the present invention, a method for channel assignment of a mobile communication system in which pairs of upstream and downstream channels are assigned to mobile station units located in a radio zone of a base station unit to carry out a two-way communication between the mobile station units, and pairs of upstream and downstream channels are assigned to the mobile station units to carry out a one-to-multiple group communication between the mobile station units, includes the steps of: detecting a vacant communication channel that exists with respect to the group communication carried out in the system; and assigning the vacant communication channel, detected in the detecting step, for a one-way communication different from the group communication.
According to one preferred embodiment of the present invention, a mobile communication system in which a central control unit assigns pairs of upstream and downstream channels to mobile station units located in a radio zone of a base station unit to carry out a two-way communication between the mobile station units, and the central control unit assigns pairs of upstream and downstream channels to the mobile station units to carry out a one-to-multiple group communication between the mobile station units, includes: a memory unit which is provided in the central control unit and stores vacant channel data indicative of the presence of a vacant channel with respect to the group communication carried out by the channel assignment of the central control unit; a broadcast message unit which is provided in the base station unit and transmits a broadcast message containing the vacant channel data, stored in the memory unit, to each of the mobile station units; a message receiving unit which is provided in one of the mobile station units and receives the broadcast message transmitted by the broadcast message unit; and a call request transmission unit which is provided in the one of the mobile station units and transmits, when the vacant channel data of the received message indicates the presence of the vacant channel, a call request for a one-way communication using the vacant channel, to the base station unit, wherein the central control unit assigns the vacant channel to the one of the mobile station units in response to a call request for a one-way communication using the vacant channel, which is sent by the one of the mobile station units, and updates the vacant channel data, stored in the memory unit, as a result of the assignment of the vacant channel.
According to one preferred embodiment of the present invention, a mobile communication system in which a central control unit accesses a home location register that manages location information of respective mobile station units, the central control unit assigning pairs of upstream and downstream channels to mobile station units located in a radio zone of a base station unit to carry out a two-way communication between the mobile station units, and the central control unit assigning pairs of upstream and downstream channels to the mobile station units to carry out a one-to-multiple group communication between the mobile station units, the mobile station units in the group communication including a transmitter mobile station unit and a receiver mobile station unit, the mobile communication system includes: a channel control unit which is provided in the central control unit and transmits, when the transmitter mobile station unit is located in the radio zone and the receiver mobile station unit is not located in the radio zone, vacant channel data, indicating that an upstream communication channel corresponding to a downstream communication channel used in the group communication is unused, to a second base station unit of the receiver mobile station, the channel control unit assigning, when a call request for a one-way communication using the upstream communication channel is received from the second base station unit, the upstream communication channel to the receiver mobile station unit; and a call request transmission unit which is provided in the receiver mobile station unit and transmits, when the vacant channel data transmitted by the channel control unit is received, the call request for the one-way communication, to the second base station unit, so that the receiver mobile station unit performs the one-way communication using the upstream communication channel.
According to one preferred embodiment of the present invention, a control unit for a mobile communication system includes: a first unit which assigns pairs of upstream and downstream channels to mobile station units located in a radio zone of a base station unit to carry out a two-way communication between the mobile station units; a second unit which assigns pairs of upstream and downstream channels to the mobile station units located in the radio zone of the base station unit to carry out a one-to-multiple group communication between the mobile station units; a memory unit which stores vacant channel data indicating the presence of a vacant channel with respect to the group communication carried out by the channel assignment of the second unit; a transmission unit which transmits the vacant channel data, stored in the memory unit, to a desired one of the mobile station units via the base station unit; and a vacant channel data unit which assigns the vacant channel to the desired one of the mobile station units in response to a call request for a one-way communication using the vacant channel, which is sent by the desired one of the mobile station units, and the vacant channel data unit updating the vacant channel data, stored in the memory unit, as a result of the assignment of the vacant channel.
According to one preferred embodiment of the present invention, a mobile station unit for use in a mobile communication system, includes: a call setup unit which establishes a call between the mobile station unit and a base station unit having a radio zone where the mobile station unit is located; a control unit which receives, when the mobile station unit is in one of a waiting condition and a receiving condition of a group communication, vacant channel data sent by the base station unit, the vacant channel data indicating the presence of a vacant channel in the group communication, the control unit storing the received channel data into a memory area of the mobile station unit, and determining whether the vacant channel exists, based on the stored channel data; and a transmission unit which transmits, when the control unit determines that the vacant channel exists, a call request for a one-way communication using the vacant channel to a central control unit via the base station unit.
In the mobile communication system of the above preferred embodiments, the base station unit transmits a broadcast message to each mobile station unit by using the downstream control channel or the downstream communication channel, and the mobile station unit can independently determine whether the vacant channel exists, and can transmit, if it exists, a call request for a one-way communication, which uses the vacant channel, to the base station unit. The call request, transmitted by the mobile station unit, is delivered to the central control unit via the base station unit. In response to the call request, the central control unit or the base station unit determines whether the vacant channel actually exists in the radio zone of the base station unit where the mobile station unit originating the call request is located. When the vacant channel exits, the central control unit assigns the vacant channel to the mobile station unit originating the call request. Then the central control unit transmits a result of the channel assignment concerning the mobile station unit to the base station unit.
Accordingly, the mobile communication system of the above preferred embodiments is effective in providing efficient use of the available frequency channels. It is not necessary to perform a preliminary communication in order to ensure the channel needed for the mobile station unit to perform a one-way communication. The mobile communication system of the above preferred embodiments can carry out a one-way communication with efficient use of the vacant channels as in the group communication performed by the conventional mobile communication system.